Composite biosorbent for treatment of waste aqueous system(s) containing heavy metals

ABSTRACT

A biosorbent composition, process of preparing and use thereof wherein the biosorbent composition comprises a chitosan-coated substrate. Useful substrates include support materials such as a ceramic support material. The biosorbent composition of the instant invention is useful in treating aqueous systems, including wastewater and aqueous waste streams, by removing undesired heavy metals.

[0001] This invention relates to a novel method of effective wastewatertreatment using a novel composite biosorbent. More in particular, thisinvention relates to a new and effective method of wastewater treatmentutilizing a chitosan-coated biosorbent to remove heavy metals fromwastewater containing the same. In one embodiment this invention relatesto a novel method for preparing a novel chitosan-coated biosorbent. Inanother embodiment this invention relates to a novel biosorbentcomposition.

[0002] In yet another embodiment, this invention presents a novelcomposite biosorbent prepared from a biological material with a highaffinity for heavy metals. A chitosan-coated biosorbent has a highaffinity for heavy metal adsorption. Chitosan is coated on to a supportmaterial, such as a ceramic support material, to provide support andstability to the biomaterial and to facilitate column flow conditionsand enhance mass transfer characteristics of the biosorbent material

BACKGROUND OF THE INVENTION

[0003] It is not uncommon for some industrial processes, businesses oreven naturally occurring geographical or weather phenomena to produceaqueous waste streams which contain one or more undesirable heavymetals.

[0004] Conventional known methods of treating such aqueous waste streamscontaminated with an undesired heavy metal(s) to remove one or more ofthe heavy metals include chemical precipitation as hydroxides orsulfides, chemical oxidation or reduction, evaporation, carbonadsorption, ion exchange, flotation, centrifugation and membraneseparate technologies. The estimated cost of the treatment for heavymetal contaminated wastewater range from $0.20-2.00 per gallonapproximately depending on the concentration and type of the metalspresent in the waste matrix. Despite the aforementioned, industries suchas metal plating and finishing industries continue to seek higherefficiency and more cost-effective technologies for treating the abovementioned aqueous streams whereby the heavy metals are removed from thewastestreams containing the same.

OBJECTS OF THE INVENTION

[0005] It is an object of this invention to provide a novel biosorbentcomposition for the effective treatment of aqueous systems containingheavy metals.

[0006] It is an object of this invention to provide an improved processutilizing a novel biosorbent composition useful for the effectivetreatment of waste streams containing heavy metals.

[0007] It is an additional object of this invention to utilize achitosan-coated support material as a biosorbent composition for theeffective treatment of waste streams.

[0008] It is an additional object of this invention to produce abiosorbent composition by utilizing a support material and coating itwith chitosan and its equivalents and the like.

[0009] It is a further object of this invention to provide achitosan-coated ceramic substrate for use as a biosorbent composition inaqueous waste streams.

[0010] It is an object of this invention to provide an environmentallyfriendly process to remove undesirable heavy metals from wastewater.

[0011] These and other objects are provided in this invention which isdescribed in more detail hereafter.

BRIEF SUMMARY OF THE INVENTION

[0012] This invention comprises a process for preparing a supportedchitosan biosorbent useful for the treatment of wastewater whichcomprises preparing a ceramic support material, preparing a chitosangel, and coating said chitosan gel to said ceramic support material.This invention further comprises a novel biosorbent composition as wellas a process for preparing and utilizing a novel biosorbent utilizingchitosan, its equivalents and the like.

[0013] This invention also comprises a biosorbent composition comprisinga support material coated with chitosan useful for treatment ofwastewater.

[0014] This invention further comprises a process for treating aqueoussystems containing heavy metals comprising adding a chitosan-coatedbiosorbent to an aqueous system.

[0015] This invention is further described in the detailed inventionwhich follows.

DETAILED DESCRIPTION OF THE INVENTION

[0016] Chitosan has a structure chemically similar to cellulose.Chitosan has the repeating units of the saccharide pictured below.

[0017] Chitosan is a biomaterial derived from the deacetylation ofchitin, a polysaccharide found in the exoskeleton of the shells ofshrimp, crab and other arthropods.

[0018] Chitosan, useful for the practice of the instant inventiondescribed herein, is available from Aldrich Chemical (Aldrich ChemicalCompany Inc, 1001 W. Saint Paul Avenue, Milwaukee Wis. 53233-2641, USA).

[0019] Chitosans high affinity for metals adsorption is well documentedin the literature. However, if chitosan is used in a flow column in itsnatural flake form, gelling and associated hydrodynamic flow problemsarise due to the fact that the metal binding sites of the chitosan arenot fully exposed for adsorption in its flake form. In order to overcomethese problems, it has now been discovered that chitosan is coated on toa support material. It has further been discovered that support materialfacilitates column flow conditions and enhances mass transfercharacteristics of the chitosan.

[0020] Support materials useful for the practice of the instantinvention are appropriately selected so that the support materialaccepts and retains the chitosan gel of the instant invention for asufficient time.

[0021] One embodiment of this invention comprises ceramic supportmaterials coated with chitosan. Nonlimiting examples of ceramic supportmaterials useful herein include alumina and silica (available fromAldrich Chemical Company, Inc., 1001 W.Saint Paul Avenue, Milwaukee,Wis. 53233-2641, USA and Sumitomo Chemical America, Inc., One CaliforniaStreet, Suit 2300, San Francisco, Calif. 94111, USA).

[0022] A preferred support material is ultra fine ceramic alumina. Suchuseful ceramic alumina is available from Aldrich Chemical Company.Ultrafine ceramic alumina means having the property of particle size inthe range of from about 10μ to about 150μ.

[0023] The process for coating chitosan on to a support material of thisinvention comprises producing a chitosan gel, producing a supportmaterial, and coating the chitosan on to the support material. Processessuch as dip coating and spin coating are useful for coating the chitosangel on to the support material.

[0024] The process of dip coating comprises the steps of preparing aceramic substrate, preparing a chitosan gel, surface coating the ceramicsubstrate with chitosan, filtration of the coated biosorbent and coatinga second surface coating of chitosan on the ceramic substrate.

[0025] A substrate to be utilized for dip coating can be prepared bydrying a selected substrate (such as a ceramic substrate) in an oven andthen storing the dried substrate in a desiccator. This substrate thencan then be mixed with an acid. After acid treatment the substrateshould be washed and dried again.

[0026] A chitosan gel to be utilized for dip coating can be prepared byadding about 3 to 10 grams of medium molecular weight chitosan to 100 mlof 10 wt % acid under constant stirring and heat addition a viscousmixture is formed.

[0027] The now prepared chitosan gel can then be coated on to the acidtreated and dried substrate by diluting the chitosan gel with waterunder heat. The acid treated substrate can then be added to the dilutedgel and stirred for about 36 hours.

[0028] After stirring, the contents are allowed to settle, and any clearliquid produced is filtered out under vacuum. The filtered material(chitosan biosorbent) should be washed with deionized water and dried inan oven at 55° C. under vacuum for 24 hours. The dried chitosanbiosorbent can then be stored for a second coat of chitosan.

[0029] A second coating of chitosan can be applied by treating the oncecoated substrate again with chitosan-acid gel under constant stirringconditions for approximately 24 hours. Again, the mixture is allowed tosettle. The supernatant liquid can then be decanted. The solid, pastymaterial recovered can then be treated with 125 ml of 1 N NaOH. Thisalkaline mixture should then be stirred again and then washed. The nowtwice-coated biosorbent should then be allowed to dry under vacuum in anoven at 55° C. for about 48 hours before transferring to a desiccatortill the biosorbent is ready for use.

[0030] The process of spin coating comprises preparation of a substrateand chitosan gel in the same manner as they would be prepared for dipcoating. The chitosan gel is then placed on to the substrate byencapsulation.

[0031] Other coating processes useful for this invention include severalmicroencapsulation processes such as coextrusion encapsulation andfluidized bed coating processes.

[0032] The chitosan of the instant invention adheres to the supportmaterial by electrostatic forces, van der Waals forces and hydrogenbonding. A nonlimiting material useful as a binding agent includesoxalic acid (available from Fischer Scientific, Fair Lawn, N.J. 07410,USA).

[0033] A chitosan-coated biosorbent has a high affinity for heavy metaladsorption. The term heavy metals, as used in this specification, isused to denote any metal which is not desirable for water, includingmetals which may be toxic to humans, animals or plants, or metals whichgive undesirable characteristics to water. Nonlimiting examples of heavymetals for which the instant invention has an affinity for includecesium, thorium, lead, mercury, arsenic and chromium mixtures thereofand the like.

[0034] The capacity of the biosorbent of the instant invention to removeheavy metals from water has been demonstrated. The chitosan biosorbentof the instant invention has a larger capacity for hexavalent chromiumcompared to that of pure chitosan or alumina by themselves.

[0035] The biosorbent of the instant invention is useful for thetreatment of wastewaters. Nonlimiting examples of wastewaters includewastewater from metal plating facilities, groundwater contaminated withhexavalanet chromium and other metals, waste waters from nuclear powerplants containing cesium, thorium and uranium, waste waters such asmercury contaminated water form dental offices, storm waters anddrinking water/waste streams contaminated with lead, mercury andarsenic.

[0036] The biosorbent of the instant invention can be added towastewater containing heavy metals or wastewater can be added to thebiosorbent of the instant invention. An effective minimum contact timeis necessary. An effective minimum contact time of about four to abouteight hours is required for maximum adsorption capacity. The minimumcontact time varies with the type of metal, concentration andtemperature.

[0037] The biosorbent of the instant invention can be added towastewater in a batch reactor to approach equilibrium conditions. Also,wastewater can be passed through a flow colunm filled with thebiosorbent of the instant invention.

[0038] The amount of any particular metal removed from aqueous streamsby unit mass of the biosorbent of the instant invention is evaluated inbatch equilibrium adsorption studies. The biosorbent of the instantinvention adsorbs two to three fold more metal ions from aqueous streamswhen tested in evaluation studies compared to chitosan in its naturalflake form.

[0039] Once heavy metals have adhered to the biosorbent of the instantinvention they can then be removed from the wastewater. Nonlimitingprocesses useful for the removal of the biosorbent of the instantinvention with heavy metals adhered thereto include regeneration of thebiosorbent loaded with heavy metals using 0.1 molar solution of sodiumhydroxide (available from Aldrich Chemical Company).

[0040] Once the chitosan with undesired heavy metals attached has beenremoved from the wastewater, the wastewater is purer than it was beforethe treatment. After treatment with the biosorbent of the instantinvention, wastewater will be purer than it would have been withouttreatment.

[0041] The biosorbent product of the instant invention is appropriatelycharacterized by electron microscopy and evaluated by the sorption ofcopper 2+, chromium 3+, chromium 6+, lead 2+, mercury 2+and nickel 2+.

[0042] Use of the Chitosan Biosorbent

[0043] Adsorption is a well-developed separation process. Thereforeextensive equipment design and development work is not required to usethe biosorbent. Traditional adsorption process equipment can be used forthe biosorbent-base processes. Any waste stream, containing metals canbe treated with the biosorbent. One such waste stream is rinse waterfrom metal plating and finishing operations. The biosorbent can also beused to treat drinking water contaminated with heavy/toxic metals suchas lead and arsenic. Special waste waters such as mercury contaminatedwater form dental offices can be treated with a cartridge filled withthe biosorbent. Waste waters for nuclear power plants containing cesium,thorium and uranium may also be treated with the biosorbent.

EXAMPLES

[0044] The examples herein are illustrations of various embodiments ofthis invention and are not intended to limit it in any way.

Example I

[0045] Overall—Preparation of Chitosan Biosorbents of This invention

[0046] A nonlimiting method to prepare the chitosan biosorbents of theinstant invention is by dip coating the ceramic substrate with chitosangel. Dip coating preparation of chitosan biosorbents involves the stepsof preparing a ceramic substrate, preparing a chitosan gel, surfacecoating the ceramic substrate with chitosan, filtration of the coatedbiosorbent and coating a second surface coating of chitosan on theceramic substrate.

[0047] A. Preparation of the Ceramic Substrate

[0048] Ceramic alumina of selected particle size was dried in an ovenfor four hours at 110° C. and stored in a desiccator. Small batches ofthe alumina were mixed with oxalic acid or acetic acid and stirred for 1hour at room temperature. The acid from the mixture was filtered andwashed twice with about 200-300 ml deionized water and filtered again.The acid treated alumina was dried in an oven at about 70° C. undervacuum for 24 hours The acid treated alumina was stored in a desiccator.

[0049] B. Preparation of Chitosan Gel

[0050] Organic acids such as acetic acid or oxalic acids can be utilizedfor dissolving chitosan. About 3 to 10 grams of medium molecular weightchitosan supplied by Aldrich Chemical is slowly added to 100 ml of the10 wt % acid while stirring constantly with a magnetic bar. The acid andchitosan formed a viscous mixture, and it was heated to 40-50° C. tofacilitate mixing. At room temperature the chitosan-acetic acid mixtureformed a transparent gel, while the chitosan-oxalic acid formed awheatish color gel.

[0051] C. Surface Coating of Ceramic Substrate with Chitosan

[0052] A known amount, about 20-40 ml of the chitosan gel was dilutedwith approximately 50 ml of water and heat to about 40-50° C. in a 250ml conical flask. About 30-50 grams of the acid treated alumina wasslowly added to the diluted gel and stirred for about 36 hours.

[0053] D. Filtration of Coated Biosorbent Material

[0054] The contents of the conical flask were then allowed to settle,and the clear liquid is filtered out under vacuum with Whatman 41 filterpaper (Whatman Inc., 9 Bridewell Place, Clifton, N.J. 07014, USA). Thefiltered material (chitosan biosorbent) was washed twice with deionizedwater and dried in the oven at 55° C. under vacuum for 24 hours. Thedried chitosan biosorbent was stored in a glass bottle for a second coatof chitosan later.

[0055] E. Second Coating of Chitosan

[0056] The once coated alumina was treated again with chitosan-oxalicacid gel, approximately 75 ml, for 24 hours under constant stirringconditions. After the 24 hours, the mixture was allowed to settle. Thesupernatant liquid was decanted and the solid, pasty material wastreated with 125 ml of 1 N NaOH. The alkaline mixture was stirred againfor two hours. Later the mixture was washed with about 250 ml deionizedwater and filtered. The twice-coated biosorbent was then allowed to dryin the oven under vacuum at 55° C. for about 48 hours. The driedbiosorbent was transferred to a glass bottle and stored in a desiccator.

[0057] A spin coating process may also be used to coat chitosan gel onto a support material such as ceramic particles.

Example II Gross and Surface Morphology

[0058] The biosorbent of the instant invention was observed under ascanning electron microscope (SEM) and a photomicroscope. Anepifluorescence photomicrograph and a couple of scanning electronmicrographs of the biosorbent are shown in FIGS. 1 and 2 respectively.These figures show gross and surface morphology of the biosorbent of theinstant invention.

Example III Equilibrium adsorption studies

[0059] Equilibrium adsorption isotherms were obtained for metals As 5+,As 3+, Cu2+, Cr6+, Ni2+, Pb2+ and Hg2+. The equilibrium experiments wereconducted at 25° C. with equilibration contact times up to about 48hours. The concentration of metals in the supernatant liquids wereanalyzed using UV/atomic absorption spectroscopy or ion chromatography.Results of the equilibrium studies are shown in FIGS. 3 and 4.

Example IV Column Studies

[0060] Column breakthrough studies with chromium, nickel, lead, mercuryand cadmium were conducted. A glass column 1 cm in diameter and 30 cm inlength with annular jacket for circulating water for maintaining thetemperature was used in this study. Results of chromium (VI) adsorptionfrom electroplating rinsewater are shown in FIG. 5. A columnbreakthrough is observed at about 15 bed volumes with the feed solutioncontaining 1253 mg/L of chromium (VI).

[0061] Thus, it is apparent that there has been provided, in accordancewith the instant invention, a process that fully satisfies the objectsand advantages set forth herein above. While the invention has beendescribed with respect to various specific examples and embodimentsthereof, it is understood that the invention is not limited thereto andmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description.Accordingly, it is intended to embrace all such alternatives,modifications and variations as fall within the spirit and broad scopeof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0062]FIG. 1 is a photomicrograph of the biosorbent of the instantinvention.

[0063] FIGS. 2(a) and 2(b) are scanning electron micrographs of thebiosorbent of the instant invention.

[0064]FIG. 3 and 4 are equilibrium evaluations of the biosorbent of theinstant invention with wastewater containing various metals.

[0065]FIG. 5 is an evaluation of the biosorbent of the instant inventionin a flow column setup.

DETAILED DESCRIPTION OF THE DRAWINGS

[0066]FIG. 1 is a photomicrograph of the composite chitosan biosorbentshowing the gross morphology. FIG. 1 is a micrograph of the biosorbentprepared with 150 mesh size alumina ceramic.

[0067] FIGS. 2(a) and 2(b) are scanning electron micrographs of thecomposite chitosan biosorbent of the instant invention. FIG. 2(a) is ascanning electron micrograph at 400×. FIG. 2(b) is a scanning electronmicrograph at 800×.

[0068]FIG. 3 is an equilibrium evaluation of the biosorbent of theinstant invention with wastewater containing copper (II), chromium (VI)and nickel (II).

[0069]FIG. 4 is an equilibrium evaluation of the biosorbent of theinstant invention with wastewater containing arsenic (III), arsenic (V),lead (II) and mercury (II).

[0070]FIG. 5 is an evaluation of the biosorbent of the instant inventionin a flow column setup. FIG. 5 depicts column adsorption of chromium(VI) from rinsewater collected from a chrome plating facility inIllinois.

What is claimed is:
 1. A process for preparing a supported chitosanbiosorbent useful for the treatment of wastewater which comprisespreparing a ceramic support material, preparing a chitosan gel, andcoating said chitosan gel to said ceramic support material.
 2. Theprocess of claim 1 wherein a second surface coating of chitosan gel isapplied to said supported chitosan biosorbent.
 3. The process of claim 1wherein said ceramic support material comprises acid treated aluminaprepared by drying ceramic alumina, mixing the dry ceramic alumina withan acid to form a mixture, filtering and washing the mixture to preparea washed mixture, and drying the washed mixture.
 4. The process of claim1 wherein said chitosan gel is prepared by dissolving chitosan in anacid.
 5. The process of claim 4 wherein said acid is an organic acid. 6.The process of claim 5 wherein said organic acid is selected from thegroup consists of acetic and oxalic acid.
 7. The process of claim 1wherein said chitosan-coated biosorbent is filtered under a vacuum,washed, dried and coated with a second layer of chitosan gel.
 8. Abiosorbent composition comprising a support material coated withchitosan useful for the treatment of wastewater.
 9. The composition ofclaim 8 wherein said biosorbent composition is useful for removing heavymetals from wastewater.
 10. The composition of claim 8 wherein saidsupport material comprises a ceramic support material.
 11. Thecomposition of claim 8 wherein said biosorbent composition is preparedby dip coating chitosan gel on to said support material.
 12. Thecomposition of claim 11 wherein said support material comprises aceramic support material.
 13. The composition of claim 10 wherein saidbiosorbent composition is prepared by spin coating chitosan gel on tosaid support material.
 14. The composition of claim 13 wherein saidsupport material comprises a ceramic support material.
 15. A process fortreating aqueous systems containing heavy metals comprising adding achitosan-coated biosorbent to an aqueous system.
 16. The process ofclaim 15 wherein said chitosan-coated biosorbent comprises a supportmaterial coated with chitosan gel.
 17. The process of claim 15 whereinsaid support material comprises a ceramic support material.
 18. Theprocess of claim 15 wherein the aqueous systems are aqueous wastestreams.
 19. A biosorbent composition comprising a support materialcoated with chitosan, its equivalents and the like useful for thetreatment for wastewater.
 20. A process for preparing a biosorbentuseful for the treatment of wastewater which comprises preparing aceramic support material, preparing a gel, and coating said gel to saidceramic support material, wherein said gel comprises chitosan, itsequivalents and the like.
 21. A process for treating aqueous systemscontaining heavy metals comprising adding a biosorbent to an aqueoussystem, wherein said biosorbent comprises a coating of chitosan, itsequivalents and the like.